REPLY
Measurement of neutralizing antibody titer against poliovirus (PV) in serum samples depends mainly on 3 factors: (i) the PV strain used as the challenge virus in the test, (ii) the use of a susceptible cell line, and (iii) the neutralization and cell culture conditions. The test is validated by the apparent titer of the challenge virus (countertitration in the test, 31 to 316 50% cell culture infectious doses [CCID50]) and the expected neutralization titer of standard serum in the tests (7). The apparent viral titer of PV strains depends on the cell culture conditions and cells, and the apparent neutralizing antibody titer of standard serum depends on the PV strain used as the challenge virus in the tests (3). Therefore, as in the much-appreciated suggestion in the letter of Arya and Agarwal commenting on our test using PV pseudovirus (1), the use of international units (IU) would be useful for the pseudovirus PV neutralization test, as well as for the conventional PV neutralization test, to standardize the tests performed among different laboratories (6). Conversion of the neutralization titer of serum samples to IU is achieved by using a conversion ratio of IU/neutralizing antibody titer of in-house standard serum used in the tests. IU values of in-house standard serum are obtained from the neutralizing antibody titers of in-house standard serum and of international standard serum with given IU values for each serotype of PV. Generally, PV neutralizing antibody titers that are equal to or more than 8 are considered protective levels against paralytic poliomyelitis (4). At present, threshold values of IU for protection against paralytic poliomyelitis have been proposed (2, 5) but have yet to be established. Nevertheless, standardization of pseudovirus PV neutralization tests by IU would be useful for serosurveillance and related biological tests for PV.
ACKNOWLEDGMENTS
The authors declare no conflict of interest and that no ethical clearance was required.
The study was supported in part by Grants-in-Aid for the Promotion of Polio Eradication and Research on Emerging and Re-emerging Infectious Diseases from the Ministry of Health, Labour and Welfare.
Footnotes
This is a response to a letter by Arya and Agarwal (doi:10.1128/CVI.05568-11).
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